Silver nanoparticles have valuable applications in the rapidly emerging field of nanomaterials. Exemplary silver nanoparticle applications include formulated biocides, antimicrobials and disinfectants, electronic chemicals, silver conductive ink, medical applications, wound care, solar panels and smart glass.
Although nanoparticles of silver in low concentration in aqueous and organic solutions are easy to prepare in a (physico-)chemical or photochemical way, their scaling-up needs a careful control of experimental conditions in order to avoid disparity from batch-to-batch. An increase in molar concentrations of the reagents generally results in an increase in particle size and agglomeration among particles. Since the benefit of nanoparticles is in their particle size, these are unwanted characteristics.
A typical chemical production process requires a dilute solution of silver salt, a surfactant or capping agent and a reducing agent. The solvent wherein the nanoparticles are produced can be water or an organic solvent such as N,N′-dimethylformamide (DMF). Most syntheses describe the use of suitable surface capping agents in addition to the reducing agents for synthesis of nanoparticles. Frequent use of organic compounds as well as polymers has been described for obtaining re-dispersible nanoparticle powders. These powders are normally post-treated by physical tempering, or alternative techniques such as thermal plasma processing, in order to obtain even smaller particles. The surface areas obtained from such methods is typically within a range not exceeding about 20 m2/g, with particle sizes of about 30 nm.
Although nanoparticles of silver in low concentrations in aqueous and organic solutions are thus easy to prepare, scale-up remains difficult in order to control the size and prevent agglomeration of silver nanoparticles. Moreover, in view of the important fields of use of silver nanoparticles in formulated biocides, antimicrobials and disinfectants, the antimicrobial efficacy of the silver nanoparticles is crucial and is closely related to the physicochemical properties of the nanoparticles.
Since nanoparticles produced in this way are generally very expensive, applications in polymers have focused on generating silver nanoparticles in situ. The in situ synthesis of silver nano-particles in polymers as host materials is well established. When nanoparticles are embedded or encapsulated in polymer, the polymer acts as surface capping agent. Polymers such as poly(vinylalcohol), poly(vinylpyrolidone), polystyrene and polymethacrylate are all suitable polymers described in literature.
However obtaining zero-valent silver of desired shape, reactivity, and size distribution within the polymer matrix remains highly challenging. Moreover, important challenges remain in this approach, such as the stability of silver nanoparticles in the polymer, as well as the prevention of aggregate formation and minimal oxidation of the polymer. Thus there is still a need in the art for improving specific physical properties, especially specific surface area characteristics and/or isoelectric point, of silver nanoparticles, as well as for improving methods for producing silver nanoparticles.